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Fair Value Measurements (Schedule of Gains and Losses Included in the Level 3 Fair Value Category) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Derivative Instruments Gain Loss [Line Items] |  |  |  |
Total gains (losses) included in earnings | $ (7) | $ (11) | $ (27) | $ (36)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets/liabilities still held at the reporting date |  | 1 |  | 1
Operating Revenue |  |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |  |
Total gains (losses) included in earnings |  | 1 | (2) | 1
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets/liabilities still held at the reporting date |  | 1 |  | 1
Electric Fuel and Other Energy-Related Purchases |  |  |  |
Derivative Instruments Gain Loss [Line Items] |  |  |  |
Total gains (losses) included in earnings | $ (7) | $ (12) | $ (25) | $ (37)

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- Definition This element represents the net amount of realized and unrealized gains or (losses) included in earnings for assets and liabilities measured at fair value on a recurring basis and included in the Level 3 fair value category. + References No definition available. + Details
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- Definition This item represents the amount of the total realized and unrealized gains or losses for the period which are included in the statement of income (or changes in net assets); the fair value of which assets and liabilities was or is measured on a recurring basis using significant unobservable inputs (Level 3). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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